EDITORIAL

Multiple products are typically formed during CO 2 hydrogenation over oxide-supported metal catalysts, making the regulation of product selectivity both practically and fundamentally important. In this work, a silver-promoted In 2 O 3 catalyst (Ag/In 2 O 3 ) was synthesized using a deposition-precipitation method and employed as a model system to elucidate the pressure-dependent reactivity of the catalyst in CO 2 hydrogenation. A strong electronic metal-support interaction (EMSI) between Ag and defective In 2 O 3 (In 2 O 3-x ) is identified, which promotes the formation of catalytically active Ag-In 2 O 3-x interfacial sites. The Ag/In 2 O 3 catalyst exhibits a 100% CO selectivity at 0.1 MPa, while achieving high methanol selectivity at 5 MPa within the temperature range investigated. Kinetic analysis reveals that CO formation and methanol synthesis share the same active sites. Combined in situ catalyst characterization and theoretical calculations demonstrate that elevated pressure enhances CO adsorption and stabilizes surface CO*, thereby promoting the further hydrogenation of CO* to CH 3 O* and ultimately leading to high methanol selectivity. This work provides mechanistic insights into pressure-controlled selectivity in catalytic CO 2 hydrogenation and offers a promising strategy for tuning product distribution via the change in the pressure. • Ag/In 2 O 3 is active for the RWGS reaction at atmospheric pressure. • 100% CO selectivity is achieved at atmospheric pressure. • An electronic metal-support interaction (EMSI) exists in Ag/In 2 O 3 . • EMSI induces partially positively charged Ag species. • Ag/In 2 O 3 shows pressure-dependent reactivity in CO 2 hydrogenation.

In recent times, perfluorinated organic compounds have been investigated extensively to understand the influence of perfluorination on structure−reactivity correlations. Here, we report the time-resolved resonance Raman (TR3), ab initio Hartree−Fock (HF), and density functional theoretical (DFT) studies on the photogenerated transient states of perfluoro-p-benzoquinone (Fluoranil, FA). In particular, for the triplet excited states, radical anion and ketyl radical Raman spectra have been recorded. The observed Raman excitation profiles and the decay rate constants of triplet excited states of FA satisfactorily reproduce, respectively, the absorption spectra and decay rate constants reported previously from transient absorption studies. The structure and vibrational spectra of all these intermediates of FA have been calculated using both ab initio unrestricted Hartree−Fock (UHF) and density functional (UBP86) methods with standard 6-31G(d) basis set. The assignments for all the experimentally observed resonance Raman bands are made using the calculated frequencies and the normal coordinate analysis. Potential energy distributions (PEDs) are also presented. Perfluoro effect is found to be more pronounced in the triplet excited state than in the ground state or the radical anion, whereas the effect in the ground state seems to be higher than that in the radical anion. The lowest triplet excited state of FA has been identified as the ππ* state (3B3G) in nonpolar solvents and the nπ* state (3B1G) in polar solvents. The solvent polarity appears to play a major role in the nature of the lowest triplet excited states, since these two states are very close to each other.

Methanol synthesis by CO and CO2 hydrogenation on Cu/γ-Al2O3 surface in liquid paraffin solution

CO2 hydrogenation to methanol is one of the main and valuable catalytic reactions applied on Cu/ZnO-based catalysts; the interface formed through Zn migration from ZnO support to the surface of Cu nanoparticle (ZnOx-Cu NP-ZnO) has been reported to account for methanol synthesis from CO2 hydrogenation. However, the accompanied reverse water gas shift (RWGS) reaction significantly decreases methanol selectivity and deactivates catalysts soon. Inhibition of RWGS is thus of great importance to afford high yield of methanol. The clear understanding of the reactivity of RWGS reaction on both the direct contact Cu-ZnO interface and ZnOx-Cu NP-ZnO interface is essential to reveal the low methanol selectivity in CO2 hydrogenation to methanol and look for efficient catalysts for RWGS reaction. Cu doped plate ZnO (ZnO:XCu) model catalysts were prepared through a hydrothermal method to simulate direct contact Cu-ZnO interface and plate ZnO supported 1 wt % Cu (1Cu/ZnO) catalyst was prepared by wet impregnation for comparison in RWGS reaction. Electron paramagnetic resonance (EPR), XRD, SEM, Raman, hydrogen temperature-programmed reduction (H2-TPR) and CO2 temperature-programmed desorption (CO2-TPD) were employed to characterize these catalysts. The characterization results confirmed that Cu incorporated into ZnO lattice and finally formed direct contact Cu-ZnO interface after H2 reduction. The catalytic performance revealed that direct contact Cu-ZnO interface displays inferior RWGS reaction reactivity at reaction temperature lower than 500 °C, compared with the ZnOx-Cu NP-ZnO interface; however, it is more stable at reaction temperature higher than 500 °C, enables ZnO:XCu model catalysts superior catalytic activity to that of 1Cu/ZnO. This finding will facilitate the designing of robust and efficient catalysts for both CO2 hydrogenation to methanol and RWGS reactions.

Tuning CO2 hydrogenation selectivity via metal-oxide interfacial sites

Supports can widely affect or even dominate the catalytic activity, selectivity, and stability of metal nanoparticles through various metal-support interactions (MSIs). However, underlying principles have not been fully understood yet, because MSIs are influenced by the composition, size, and facet of both metals and supports. Using Ru/TiO2 supported on rutile and anatase as model catalysts, we demonstrate that metal-support interfacial compatibility can critically control MSI modes and catalytic performances in CO2 hydrogenation. Annealing Ru/rutile-TiO2 in air can enhance CO2 conversion to methane resulting from enhanced interfacial coupling driven by matched lattices of RuOx with rutile-TiO2; annealing Ru/anatase-TiO2 in air decreases CO2 conversion and converts the product into CO owing to strong metal-support interaction (SMSI). Although rutile and anatase share the same chemical composition, we show that interfacial compatibility can basically modify metal-support coupling strength, catalyst morphology, surface atomic configuration, MSI mode, and catalytic performances of Ru/TiO2 in heterogeneous catalysis.

The synthesis of methanol from CO2 hydrogenation is an effective measure to deal with global climate change and an important route for the chemical fixation of CO2. In this work, carbon-confined MoSe2 (MoSe2@C) catalysts were prepared by in situ pyrolysis using glucose as a carbon source. The physico-chemical properties and catalytic performance of CO2 hydrogenation to yield methanol were compared with MoSe2 and MoSe2/C. The results of the structure characterization showed MoSe2 displayed few layers and a small particle size. Owing to the synergistic effect of the Mo2C-MoSe2 heterojunction and in situ carbon doping, MoSe2@C with a suitable C/Mo mole ratio in the precursor showed excellent catalytic performance in the synthesis of methanol from CO2 hydrogenation. Under the optimal catalyst MoSe2@C-55, the selectivity of methanol reached 93.7% at a 9.7% conversion of CO2 under optimized reaction conditions, and its catalytic performance was maintained without deactivation during a continuous reaction of 100 h. In situ diffuse infrared Fourier transform spectroscopy studies suggested that formate and CO were the key intermediates in CO2 hydrogenation to methanol.

Vibrational and electronic absorption spectroscopic and density functional theoretical studies on the 2(3H)-benzothiazolone and its anion

This perspective focuses on the modulation of metal-support interaction (MSI) in catalysts for COx hydrogenation, highlighting their profound impact on catalytic performance. Firstly, it outlines different strategies, including the use of highly reducible oxides and moderate reduction treatments, which induce the classical strong metal-support interaction (SMSI) effect and the electronic metal-support interaction (EMSI) effect. Morphology engineering and crystalline phase manipulation of oxides presented as effective methods to control EMSI are also discussed. The discrimination of SMSI and EMSI can be achieved using oxides with low encapsulation tendencies, such as ZrO2, which supports electronic modifications without or minimizing the overgrowth issues, optimizing the catalytic performance for methanation. Then, the synergy between Cu and ZnO in methanol synthesis, enhanced by SMSI, is emphasized inside. Optimizing support oxides to control oxygen vacancies enhances the catalytic performance of CO2 hydrogenation to methanol. Perspectives for the future research on the fundamental understanding of structure-MSI-performance relationship for catalyst design is discussed.

A density functional theory study of CO2 hydrogenation to methanol over Pd/TiO2 catalyst: The role of interfacial site

Excellent low-temperature activity for oxidation of benzene serials VOCs over hollow Pt/CoMn2O4 sub-nanosphere: Synergistic effect between Pt and CoMn2O4 on improving oxygen activation

Improving methanol selectivity in CO2 hydrogenation by tuning the distance of Cu on catalyst

Open AccessCCS ChemistryRESEARCH ARTICLES22 Oct 2022Decrypting the Influence of Axial Coordination on the Electronic Microenvironment of Co-N5 Site for Enhanced Electrocatalytic Reaction Bingyu Huang†, Senhe Huang†, Chenbao Lu, Longbin Li, Judan Chen, Ting Hu, Dirk Lützenkirchen-Hecht, Kai Yuan, Xiaodong Zhuang and Yiwang Chen Bingyu Huang† Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Senhe Huang† Themeso-Entropy Matter Lab, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 , Chenbao Lu Themeso-Entropy Matter Lab, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 , Longbin Li Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Judan Chen Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Ting Hu School of Materials Science and Engineering, Nanchang University, Nanchang 330031 , Dirk Lützenkirchen-Hecht Faculty of Mathematics and Natural Sciences-Physics Department, Bergische Universität Wuppertal, Wuppertal D-42119 , Kai Yuan *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 , Xiaodong Zhuang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Themeso-Entropy Matter Lab, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 and Yiwang Chen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Institute of Polymers and Energy Chemistry (IPEC), College of Chemistry and Chemical Engineering, Nanchang University, Nanchang 330031 Institute of Advanced Scientific Research (iASR), Key Lab of Fluorine and Silicon for Energy Materials and Chemistry of Ministry of Education, Jiangxi Normal University, Nanchang 330022 https://doi.org/10.31635/ccschem.022.202202241 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Metal porphyrins are star molecules that possess well-defined coordination metal centers for versatile catalytic reactions. However, most previous work has focused on the correlations between in-plane symmetric configuration of metal-N4 sites and their catalytic performance. Addressing the catalytic contribution of additional axial coordination to such symmetric configuration remains a challenge. Theoretical calculations revealed that axially anchoring an extra pyridine on the tetra-coordinated cobalt porphyrin (Co-N4) to construct penta-coordinated cobalt porphyrin (Co-N5) renders cobalt a higher electron density, thereby favoring the rate-determining O2 adsorption/activation and reducing the oxygen electroreduction barrier. Therefore, a well-defined Co-N5 site is rationally introduced into the azo-linked polymer framework for a fundamental structure–catalytic performance correlation study. As-prepared Co-N5 catalyst exhibits a 26 mV positive shift in half-wave potential compared with the pyridine-free Co-N4 counterpart, discloses a markedly higher power density (141.4 mW cm−2), and possesses better long-term durability (over 160 h cycles) in a Zn-air battery. Moreover, such a Co-N5 catalyst also showcases potential applications for CO2 reduction with high CO2-to-CO conversion faradic efficiency and better selectivity than the Co-N4 counterpart because coordination of the fifth pyridine evokes electronic localization that suppresses a competitive side reaction. This work proves the positive electrocatalytic contribution of axial penta-coordination on well-defined metal-porphyrin-based catalysts and offers atomic understanding of the structure–performance correlation on single atom catalysts for future catalyst design. Download figure Download PowerPoint Introduction Currently, many advanced electrocatalysts have been developed to facilitate the sluggish kinetics of the multiple-electron transfer process in energy conversion reactions, such as the oxygen reduction reaction (ORR) and CO2 reduction reaction (CO2RR).1–4 Due to maximized atom utilization, mainstream research has focused on the isolated transition metal/nitrogen coordinated (M-N-C) single-atom catalysts (SACs) with high catalytic activity and selectivity.5–9 Despite extensive investigations, the rational design and controllable and precise synthesis of M-N-C catalysts continues to be the main obstacle to multiple-electron transfer processes.10–12 The preparation of M-N-C catalysts normally needs high-temperature pyrolysis to increase the graphitization degree for better conductivity.13–17 Unfortunately, the pyrolysis process inevitably evokes metal aggregation due to the thermal decomposition of metal precursors and high surface energy of single metal atoms, posing challenges for maintaining atomic-metal isolation.13,18,19 The inhomogeneity and indistinction of the catalytic environment of carbonaceous materials give rise to tremendous difficulties in simultaneously enhancing the catalysts' activity and selectivity. In addition, the inherently less-defined active sites formed after pyrolysis leaves an ambiguous structure–performance relationship and seriously precludes us from exploring the in-depth mechanism for different electrocatalytic reactions.20–24 Therefore, the above-mentioned challenges stimulate the vigorous search for developing cost-effective and high-performance pyrolysis-free electrocatalysts with well-defined active sites. Transition-metal macrocycles, such as cobalt porphyrins, which possess the distinct tetra-coordinated cobalt porphyrin (Co-N4) site, have been heavily studied.25–28 Benefitting from the production of fewer radical oxygen species during the electrocatalytic process, the Co-N4 structure is more robust and advantageous for electrocatalysis of ORR.16,20,29–31 However, the representative plane-symmetric electron configuration of Co-N4 is not the optimal structure for the chemisorption and activation of reactants.32–34 Breaking the structure symmetry with penta-coordination to regulate the charge redistribution of Co-N4 sites could promote the electrocatalytic process, but the accurate synthesis of this structural motif is still a noteworthy challenge. Besides, the role of penta-coordination and the exact local microenvironment of such active sites have not yet been ascertained.35,36 In view of this, offering a model catalyst as an ideal platform is urgently required to elucidate the contribution of axial penta-coordination on catalytic activity, selectivity, and durability of Co-N4 sites. In this work, we probed the positive effect of axial pyridinic penta-coordinated cobalt porphyrin (Co-N5) to achieve optimized electronic localization on the Co-N5 site for boosting ORR through density functional theory (DFT) calculations as well as electrochemical analysis. DFT calculations indicate Co-N5 with axial pyridinic coordination possesses obviously higher electronic density on the Co center in comparison with Co-N4. Taking advantage of the electronic-push effect of penta-coordination, the ORR rate-determining step of O2 adsorption/activation can be significantly promoted, ensuring better ORR performance of Co-N5 than the Co-N4 model. Hence, we innovatively designed and synthesized an azo-linked polymer framework with atomically dispersed electron-rich Co-N5 sites for ORR through a pyrolysis-free axial-pyridinic coordination strategy. The Co-N5 catalyst displays impressive ORR performance with a more positive half-wave potential of 0.811 V and lower Tafel slope of 39 mV dec−1 than the Co-N4 counterpart (0.785 V and 50 mV dec−1), which support the theoretical predictions. Encouragingly, such a penta-coordination-induced electronic localization strategy also promises the potential for improving the CO2-to-CO conversion faradic efficiency and selectivity of electrocatalytic CO2 reduction. This work provides new design strategies toward well-defined single-atom electrocatalysts with axial coordination and offers new models for fundamentally understanding the catalytic mechanism of asymmetric coordination systems. Experimental Methods Synthesis of [email protected] catalysts First, 20 mg (27.3 μmol) of CoTAPP and 80 mg of G-py were redispersed in 20 mL of dimethyl sulfoxide (DMSO). Then, the mixture was stirred for 15 h to ensure CoTAPP was fully coordinated to G-py. Immediately after the coordination, 35.2 mg (0.11 mmol, 4 equiv to CoTAPP) of PhI(OAc)2 was added to the mixed solution, and the mixture was stirred for 24 h. The resultant precipitate was filtered and washed with DMSO, methanol, deionized water, and ethanol for three times, and dried at 60 °C overnight. [email protected] was harvested as a black powder (86.8 mg, 87% yield). [email protected] and [email protected] were prepared with a procedure similar to [email protected] Experimental details, materials characterization methods, and synthesis of other samples are available in the Supporting Information. Results and Discussion Theoretical calculations and catalytic mechanism Based on cobalt porphyrin, two models with different coordination environments, axial pyridinic Co-N5 model and Co-N4 model were designed (Figure 1a and Supporting Information Figure S1). The elementary steps and corresponding ORR adsorption configurations on the two models are presented in Supporting Information Figure S2. The Gibbs free energies at different potentials for all elementary steps were evaluated to illustrate how the penta-coordination affects intrinsic ORR activity. As given in Figure 1b, all elementary steps are distinctly downhill at U = 0 V, thus the reaction is exothermic and able to proceed spontaneously. When the potential rises to 1.23 V, the rate-determining step of Co-N4 is the first step (* + O2 + H2O + e− → *OOH + OH−) with a high energy barrier of 0.34 eV. In contrast, the final release step (*OH + e− → OH− + *) of the Co-N5 model is the rate-determining step, and its free energy can be distinctly reduced to only 0.26 eV. The thermodynamic limiting potentials, which represent the maximum potential to ensure all steps downhill are 0.97 and 0.89 V for Co-N5 and Co-N4 models, respectively (Figure 1c and Supporting Information Figure S3), revealed the Co-N5 model requires the minimum overpotential to drive the oxygen reduction. This result implies that the axial pyridine coordination plays a crucial role in regulating the ORR intermediates adsorption strength and decreasing the reaction barrier of the rate-determining step. Similarly, axial pyridine coordination can also alter the Fe electronic microenvironment, thereby enhancing the catalytic activity of the Fe-N5 model ( Supporting Information Figures S4–S7), further validating the universality of additional axial coordination. Figure 1 | (a) DFT calculation models of Co-N5 and Co-N4 for the electrochemically catalyzed ORR. (b) Free energy diagrams at U = 0 V and U = 1.23 V, and (c) free energy diagrams for the thermodynamic limiting potentials of Co-N5 model and Co-N4 model. (d) PDOS of Co atom for Co-N5 model (top) and Co-N4 model (bottom); the d-band center is denoted by the dashed gray line. Differential charge density distribution after O2 absorption on (e) Co-N5 model and (f) Co-N4 model. Download figure Download PowerPoint To further study axial coordination induced changes in the Co center's electronic configuration and interaction with oxygen-containing intermediates, projected density of states (PDOS) was conducted. The axial coordination obviously tunes the Co 3d orbital according to the PDOS in Figure 1d. The d-band center of the Co-N5 model at −1.59 eV is closer to the Fermi level than that of Co-N4 at −1.76 eV, thereby leading to an increase in O-containing intermediates' adsorption.37 The enhanced adsorption ensures subsequent ORR steps proceed through a more efficient four-electron pathway. Hence, the thermodynamic onset potentials improvement can be ascribed to fine-tuned adsorption strength of the ORR intermediates, which directly determines the activity and selectivity of catalysts. In addition, the higher PDOS near the Fermi level represents more abundant charge carriers and better electronic conductivity for the Co-N5 model. During the ORR process, the activated Co d orbitals and their hybridization with O p orbitals co-determine the adsorption strength for oxygen-containing adsorbates.38–42 After O2 adsorption and *OH formation, the increased overlapping degree of the strong σ-bond that originates from the Co dz2 orbital and O p orbital, along with the decreased O2 antibonding orbital filling degree that appears above the Fermi level for Co-N5 model, can be observed. It theoretically suggests that the extra fifth pyridine coordination assures a tighter connection between Co centers and O2 ( Supporting Information Figures S8 and S9), thus enabling higher ORR selectivity towards the four-electron pathway.43 As verified by charge density differences ( Supporting Information Figure S10), obvious asymmetrical charge distribution caused by axial coordination can be found for the Co-N5 model in comparison with the symmetric Co-N4 model. An apparent charge accumulation on the Co center is found to form the electron-rich Co-N5 site due to the electronic-push effect of axial pyridine. As expected, the O2 only absorbs on the individual Co center (Figure 1e,f), which can provide superior ORR electrocatalytic sites. Furthermore, the axial pyridine can construct an electronic pathway that renders adequate charge transfer to oxygen molecules from the conductive graphene layer. In general, the electron-rich Co-N5 site promises stable chemisorption and activation of O2, which facilitates O–O bond cleavage, thus offering better selectivity for the four-electron reduction pathway. Therefore, the axial-pyridine coordination-induced electronic localization strategy is viable to efficaciously enhance the ORR kinetics. Synthesis and structural characterization To experimentally confirm the calculation results and demonstrate the significance of penta-coordination architecture in ORR, an azo-linked penta-coordinated cobalt porphyrin-based polymer (CoTAPP-Azo) anchored on pyridine functionalized graphene (G-py) ([email protected]) was synthesized. The synthesis route and structure of [email protected] and the counterpart [email protected] (directly grown CoTAPP-Azo on pristine graphene) are schematically revealed in Figure 2a. Tetrakis(4-aminophenyl) porphyrin (TAPP) was first chelated with cobalt cation to obtain CoTAPP ( Supporting Information Figure S11). TAPP coordinating with Fe (FeTAPP) and Ni (NiTAPP) were also successfully obtained using the same procedure. The formation of CoTAPP, FeTAPP, and NiTAPP were confirmed by mass spectrometry ( Supporting Information Figure S12) and Fourier-transform infrared (FTIR) spectroscopy ( Supporting Information Figure S13). The successful pyridine functionalization in G-py was confirmed by thermogravimetric analysis ( Supporting Information Figure S14), Raman spectroscopy ( Supporting Information Figure S15), and X-ray photoelectron spectroscopy (XPS) ( Supporting Information Figure S16). Subsequently, CoTAPP was anchored on G-py through axial pyridine coordination; following an azo-coupling reaction, [email protected] was finally obtained (see details in Supporting Information Scheme S1). For comparison, FeTAPP-based polymer hybridized with G-py ([email protected]) and graphene ([email protected]) and NiTAPP-based polymer hybridized with G-py ([email protected]), were synthesized by similar routes. Figure 2 | (a) Synthetic route of CoTA[email protected]y and [email protected] (b) HAADF-STEM image of [email protected] (c) High-resolution Co 2p XPS spectra of [email protected] and [email protected] Download figure Download PowerPoint The morphology of [email protected] was first identified by scanning electron microscopy (SEM). Compared with the thin-layered sheet structure of G-py, a thicker layer was observed, indicating G-py was covered by CoTAPP-Azo ( Supporting Information Figures S17 and 18). The transmission electron microscopy (TEM) image of [email protected] also presented representative lamellar plate morphology ( Supporting Information Figure S18c). To further discern the structural features at the atomic level, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was carried out. HAADF-STEM images and the corresponding energy dispersive spectroscopy (EDS) of [email protected] indicated homogeneous spatial distribution of C, N, Co elements ( Supporting Information Figure S19). The abundant bright points (highlighted by red circles) offer direct evidence for the uniform distribution of single Co atoms (Figure 2b). Similarly, [email protected], [email protected], and [email protected] all exhibited similar lamella morphology with homogeneously distributed elements ( Supporting Information Figures S20–S22). The chemical structures of [email protected] were preliminarily examined by FTIR spectroscopy ( Supporting Information Figure S23). Compared with CoTAPP, the corresponding FTIR spectrum of [email protected] displayed a significant intensity increase of N=N stretching signals (1570 cm−1, 1215 cm−1) along with an obviously weakened N–H peak at 3400 cm−1, proving the successful polymerization of CoTAPP. The increased ID/IG ratio of 0.94 for [email protected] to 1.07 for [email protected] in the Raman spectra ( Supporting Information Figure S24) verifies surface lattice interference caused by penta-coordination. X-ray diffraction (XRD) confirmed that G-py was coated by CoTAPP-Azo, and the absence of crystalline cobalt species diffraction peaks demonstrated the highly dispersed state of Co atoms ( Supporting Information Figure S25), agreeing well with the HAADF-STEM observations. [email protected] and [email protected] both show similar XRD patterns ( Supporting Information Figures S26 and S27). Furthermore, XPS was extracted to unravel the nature of the chemical bonding of [email protected] and [email protected] ( Supporting Information Figures S28–S31). Compared with [email protected], the N 1s XPS spectrum for [email protected] displayed an obvious pyridinic N peak (398.3 eV, Supporting Information Figure S29). The C 1s XPS spectrum of [email protected] showed that the C–N peak shifts to higher binding energy (BE) than that of [email protected] (ΔBE = 0.4 eV), revealing the decreased electron density of C atom. In the Co 2p spectra (Figure 2c), [email protected] underwent a shift to lower BE for Co 2p3/2 (780.3 eV) and Co 2p1/2 (795.6 eV) peaks, relative to that of [email protected] (780.8 and 796.0 eV for Co 2p3/2 and Co 2p1/2, respectively), providing evidence for the electronic localization on the Co with penta-coordination.44,45 This result confirms that axial pyridine ligands act as channels between G-py and CoTAPP-Azo and boost the charge transfer from graphene to Co centers, which is consistent with the electron-rich Co-N5 model in theoretical calculations. Importantly, similar phenomena were found in [email protected] and [email protected], suggesting the universality of such an electron localization approach. The UV–vis spectrum of [email protected] revealed a clear porphyrin Soret band at 448 nm ( Supporting Information Figure S32). Interestingly, an appreciable peak change to 438 nm in [email protected] was observed due to the penta-coordination-induced electron transfer. Meanwhile, photoluminescence spectroscopy was conducted to explore the charge separation behaviors ( Supporting Information Figure S33). Significantly increased quenching occurred in [email protected] compared with [email protected], further demonstrating that axial pyridine can enhance the charge transfer between G-py and CoTAPP-Azo. The work function, which represents the minimum energy needed to draw one inner electron from the nucleus, was obtained by ultraviolet photoelectron spectroscopy. [email protected] showed a 0.51 eV shift to higher BE than [email protected] in the second electron cut-off edge ( Supporting Information Figure S34). Hence, with respect to 4.49 eV for [email protected], the smaller work function for [email protected] (3.98 eV) indicated that the electrons are more likely to be activated and transferred outward.46,47 We assessed the porosity of the samples by nitrogen physisorption isotherms ( Supporting Information Figure S35). [email protected] and [email protected] both displayed the typical type-IV isotherms with an evident hysteresis loop, indicative of the coexistence of micropores and mesopores. The Brunauer–Emmett–Teller specific surface areas for [email protected] and [email protected] were calculated to be 506.6 and 440.3 m²/g, respectively. Pore size distribution demonstrated by nonlocalized DFT showed the pore widths centered at 1.9 and 4.8 nm for [email protected] ( Supporting Information Figure S36). These results demonstrate the improved surface area and hierarchical porous structure of [email protected], which are conducive to increase the quantity of accessible active sites and enhance the mass transport. To investigate the local coordination environment of the Co center, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses were examined. The stronger white-line peak intensity of [email protected] (7727 eV) compared with [email protected] was assigned to the larger coordination number of the Co center for [email protected] than [email protected] (Figure 3a).48 The quantitative fits ( Supporting Information Table S1) showed a Co atom in [email protected] was straightforwardly connected by N atoms with a coordination number of 5.1 and the average bond length of 1.98 Å. From the Co K-edge Fourier-transformed EXAFS spectra (Figure 3b), the first shell peak for [email protected] around 1.43 Å corresponded to the Co–N scattering path, accompanying the absent Co–Co peak at 2.18 Å, jointly signifying the isolated Co atoms' configuration. [email protected] displayed an obviously positive shift for the Co–N first shell in than [email protected], suggesting an Co–N bond length caused by the to the EXAFS results (Figure and Supporting Information Figures and in to an of for [email protected], the first shell intensity for [email protected] was enhanced with an to revealing the larger number of N coordination The was also for the coordination In Figure the intensity maximum at for [email protected] was assigned to Co–N Co–Co and corresponding coordination was demonstrating the of the Co The shift of [email protected] maximum compared with that of CoTAPP is caused by the in [email protected] Hence, on and EXAFS as well as the XPS we that [email protected] by Co-N5 sites was successfully synthesized. Figure | (a) Co K-edge spectra and (b) the of Co K-edge EXAFS spectra of [email protected] with Co and CoTAPP as (c) The corresponding EXAFS of [email protected] model with Co N and C (d) of [email protected] in comparison with Co and CoTAPP. Download figure Download PowerPoint Electrocatalytic ORR performance To how axial penta-coordination affects ORR [email protected] and [email protected] were first evaluated with a In ( Supporting Information Figure significant peaks in for [email protected], but a peak was observed in The (Figure and Supporting Information Figure that [email protected] the ORR performance with the most positive half-wave potential of 0.811 V which is 26 mV more positive than that of [email protected] (0.785 V These results indicate that [email protected] possesses superior catalytic performance that from the electron localization caused by pyridine penta-coordination. in comparison with [email protected], the density for [email protected] was enhanced because the axial pyridine as between G-py and CoTAPP-Azo, thus boosting the charge transfer from graphene to CoTAPP layer. Figure 4 | (a) ORR of [email protected] and [email protected] catalysts at (b) Tafel (c) density by electrochemically active surface and (d) comparison of Tafel and for [email protected] and [email protected] catalysts. (e) The of [email protected] and [email protected] and after (f) and power density specific for Zn-air at and long-term performance of Zn-air at a density of using [email protected], [email protected], and as catalysts. Download figure Download PowerPoint To further into the reaction kinetics of [email protected], the at from to were ( Supporting Information Figure The ( Supporting Information Figure which were from at different potentials, revealed the toward the of Based on the the number for [email protected] was which with the This proves that the Co-N5 site a four-electron pathway towards ORR, [email protected] has a smaller of ( Supporting Information Figure We the enhanced ORR selectivity to the Co-N5 configuration that can be with because of the effect between Co and pyridinic the resultant O–O bond with stronger stretching is thus providing higher selectivity for four-electron ORR. [email protected] has a more positive than [email protected] ( Supporting Information Figures that ORR activity can be improved by microenvironment through extra axial penta-coordination on different metal Moreover, the Tafel slope of 39 mV dec−1 the process of [email protected] for oxygen reduction (Figure and Supporting Information Figure by the of electrochemical spectroscopy ( Supporting Information Figure the decreased in the for [email protected], indicating the optimized and electron toward ORR kinetics. In addition, [email protected] exhibited the electrochemically active surface area ( Supporting Information Figure which is conducive to more active sites. density with an in the ORR can be found (Figure the intrinsic activity for [email protected] For further comparison, the performance density Tafel density and for different electrocatalysts are (Figure and

Catalytic CO(2) hydrogenation to methanol has received considerable attention as an effective way to utilize CO(2). In this paper, density functional theory was employed to investigate the methanol synthesis from CO(2) and H(2) on a Mo(6)S(8) cluster. The Mo(6)S(8) cluster is the structural building block of the Chevrel phase of molybdenum sulfide, and has a cagelike structure with an octahedral Mo(6) metallic core. Our calculations indicate that the preferred catalytic pathway for methanol synthesis on the Mo(6)S(8) cluster is very different from that of bulklike MoS(2). MoS(2) promotes the C-O scission of H(x)CO intermediates, and therefore, only hydrocarbons are produced. The lower S/Mo ratio for the cluster compared to stoichiometric MoS(2) might be expected to lead to higher activity because more low-coordinated Mo sites are available for reaction. However, our results show that the Mo(6)S(8) cluster is not as reactive as bulk MoS(2) because it is unable to break the C-O bond of H(x)CO intermediates and therefore cannot produce hydrocarbons. Yet, the Mo(6)S(8) cluster is predicted to have moderate activity for converting CO(2) and H(2) to methanol. The overall reaction pathway involves the reverse water-gas shift reaction (CO(2) + H(2) --> CO + H(2)O), followed by CO hydrogenation via HCO (CO + 2H(2) --> CH(3)OH) to form methanol. The rate-limiting step is CO hydrogenation to the HCO with a calculated barrier of +1 eV. This barrier is much lower than that calculated for a comparably sized Cu nanoparticle, which is the prototypical metal catalyst used for methanol synthesis from syngas (CO + H(2)). Both the Mo and S sites participate in the reaction with CO(2), CO, and CH(x)O preferentially binding to the Mo sites, whereas S atoms facilitate H-H bond cleavage by forming relatively strong S-H bonds. Our study reveals that the unexpected activity of the Mo(6)S(8) cluster is the result of the interplay between shifts in the Mo d-band and S p-band and its unique cagelike geometry.

In solid acid catalysis, understanding the impact of support acidity on catalytic performance has remained a controversial issue. The selected catalytic systems often rely on mixing different substances to control the degree of acidity, which in turn, also modifies other parameters in the system, making it challenging to perform a definitive study. To specifically investigate the role of support acidity, we performed a systematic study employing Nb2O5 as the catalyst support, which acidity can be controlled by calcination. The catalytic behavior of the fabricated Pt/Nb2O5 catalysts was evaluated using CO2 hydrogenation to methanol (MeOH) and dimethyl ether (DME). An increase in the acidity of the support resulted in an improvement in the CO2 conversion owing to the strong interaction between the Pt and the catalyst support, but it was detrimental for the production of MeOH because of the unfavorable adsorption of CO2 molecules and the formation of carbon-containing species on the surface of the support with high acidity. DME selectivity was enhanced with an increase in catalyst acidity, confirming the role of solid acids for the production of DME from CO2 reduction. By controlling the calcination temperature of Nb2O5, tunable support acidity was obtained. CO2 conversion increased while the selectivity of methanol and dimethyl ether decreased with increasing support acidity.